The present invention relates to a class of crystalline and grindable prepolymers which polymerize to a soft, low modulus, essentially non-crystalline thermosetting polyurethane.
While polyurethane prepolymers are known, and while it is also known to prepare polyurethane prepolymers by end blocking the isocyanate functionality with a heat reversible end blocking agent, conventional prepolymers have not been grindable and, at the same time, polymerizable to soft, flexible, but durable polyurethane films.
Griswold, U.S. Pat. No. 3,846,378 discloses a process for producing polyurethane prepolymers by a so called synchronous partial blocking process wherein the prepolymers are end-blocked with an oxime by mixing the oxime and a long chain polyol and reacting the mixture with an organic polyisocyanate in the presence of a polymerization catalyst. These prepolymers, however, do not appear to be good film formers or capable of being ground to a powder for use in powder coating.
Thus, there is a need, for a grindable prepolymer which provides a soft, elastic, low modulus polyurethane.